This research project is designed to explore the processes directing the evolution of self-incompatibility (S-) loci in flowering plants and major histocompatibility (MHC) loci in vertebrates. Our recent surveys of molecular-level variation at S-loci in natural populations revealed strikingly different phylogenetic patterns among S-alleles within two solanaceous species, Solanum carolinense and Physalis crassifolia. Theoretical, empirical, and statistical studies will be conducted to characterize and interpret the differences in genealogical pattern exhibited by natural populations. Methods for describing genealogical pattern will be developed. This approach offers a means of the testing deviations in tree shape from expectation. It also provides a diagnostic tool for qualitative recognition of the hallmarks of evolutionary and ecological processes. Theoretical studies will investigate the effects of inbreeding depression, clonal reproduction and extinction, and population structure on the evolution of genealogical relationships among S-alleles. These processes may have given rise to the patterns observed in natural populations. Empirical studies will directly address whether such processes are operating in natural populations. To explore whether ecology contributes to the observed differences in genealogical pattern, S- allele variation will be surveyed in natural populations of a species that shares close ancestry with P.crassifolia but its ecology with S.carolinense. Isozyme variation will be surveyed in order to characterize the structure of natural populations. Statistical studies will be conducted to estimate the age of the form of gametophytic self-incompatibility expressed in the Solanaceae. The process of reversion of self-incompatibility to self-compatibility will be addressed through sequence analysis of a family homologous to the S- locus in a self-compatible member of the Brassicaceae.